1. Field of the Invention
This invention relates to a sampling rate converting method and apparatus for converting by filtering the sampling frequency of a 1-bit digital data equal to 64 times 44.1 Khz or the sampling frequency of 1-bit digital data equal to 49 times 48 kHz.
2. Description of the Related Art
The method for digitizing speech signals for recording, reproduction and transmission has hitherto been practiced on a recording/reproducing apparatus having an optical disc, such as a compact disc (CD), a magnetic tape, such as a digital audio tape (DAT), or in digital broadcasting such as satellite broadcasting. In digitizing the speech signals in such digital audio data transmitting method, it has been prescribed to use formats 48 kHz or 44.1 kHz as the sampling frequency and 16 or 20 bits as the numbers of quantization bits.
As another method for digitizing the speech signals, a method known as a sigma-delta (.SIGMA..DELTA.) has been proposed, as shown in Yoshio Yamazaki, "AD/DA Converter and Digital Filter", Journal of Japan Society of Acoustics, vol. 46, No. 3 (1990) pages 251 to 257.
An arrangement of a (.SIGMA..DELTA.) modulator for performing the (.SIGMA..DELTA.) modulation of, for example, one bit, is shown in FIG. 1, in which an input audio signal at an input terminal 91 is supplied via an adder 92 to an integrator 93. The signal from the integrator 93 is fed to a comparator 94 so as to be compared to, for example, the neutral potential of the input audio signal and quantized by, for example, one-bit quantization each sampling period. The frequency of the sampling periods, that is the sampling frequency, is selected to be 64, 49 or 128 times the conventional sampling frequency of 48 kHz or 44.1 kHz.
These quantization data are fed to a one-sample delay unit 96 so as to be delayed by one sampling period. The delayed data is converted by, for example, a 1-bit D/A converter 95 into analog signals which are supplied to the adder 92 so as to be added to the input audio signal from the input terminal 91. Output quantized data from the comparator 94 is taken out at an output terminal 97. Thus, with the .SIGMA..DELTA. modulation by the .SIGMA..DELTA. modulator, an audio signal having a wide dynamic range can be obtained, even with the number of bits as small as one bit, by using a sufficiently high sampling frequency. In addition, the frequency range that can be transmitted can be increased.
In addition, the .SIGMA..DELTA. modulation has a circuit arrangement that lends itself to integration and can achieve high precision in A/D conversion relatively easily, so that it is frequently used within an A/D converter. The .SIGMA..DELTA. modulated signals can be passed through a simple analog low-pass filter for restoration to the analog audio signal. Therefore, by exploiting these features, the .SIGMA..DELTA. modulator can be applied to a recorder handling high-quality data or to data transmission.
Meanwhile, should it become necessary to convert the 1-bit digital data obtained by the .SIGMA..DELTA. modulator into digital data having the sampling frequency of 32 kHz or 48 kHz, accurate conversion has so far not been achieved due to jitter or the like.
For example, if the sampling frequency of 1-bit digital data, which is 64 times 32 kHz, is to be converted into a sampling frequency of 48 kHz, the result of calculations gives 32000.times.64/48000=42.6 . . . which is not a whole number, so that accurate conversion is not feasible.
On the other hand, if the sampling frequency of 1-bit digital data, which is 64 times 48 kHz, is to be converted into a sampling frequency of 44.1 kHz, the result of calculations gives 48000.times.64/44100=69.65 . . . which is not a whole number, so that accurate conversion is again not feasible.
The same holds for the case of converting the sampling frequency of one-bit digital data equal to 64 times 96 kHz into 44.1 kHz.